Irrelevant operators increase the branching factor b, and this
rapidly increases the size of the search space, bd.

A human chess master considers only 1 to 3 board positions per second,
but considers only at the most useful moves. A computer chess
program may consider millions of positions per second, but most of those
are foolish moves.

Moral: Use knowledge about the problem domain to reduce
the number of operators considered at any given time.